The use of orthopedic staples, or bone staples, is a common method of bone fixation in orthopedic surgery. A particular type of bone staple is known as the memory staple. Memory staples are comprised of a nickel/titanium alloy, also known as Nitinol. Nickel/titanium alloy is unique in that it undergoes a phase transformation in its crystal structure, changing between the stronger austenite form to the weaker martensite form, when exposed to certain environmental stimuli, e.g., heat or stress. Memory metal made from nickel/titanium alloy is categorized as either shape memory or super elastic. Shape memory metal responds to changes in temperature whereas super elastic metal responds to stress or force applied to the metal. When a shape memory alloy is at cooler temperatures, it is in its martensitic form. The martensitic form is easily deformed to a new shape. However, when the alloy is heated through its transformation temperatures, it reverts to austenite and recovers its previous shape with great force.
Bone staples are generally U-shaped, having a bridge and first and second legs extending from respective sides of the bridge. Bone staples formed from shape memory nickel/titanium alloy are formed at high temperatures with the legs of the staple angled inwardly. The staples are then cooled and stored with the staple legs maintained in an open position, roughly perpendicular to the staple bridge. When the shape memory staples are inserted into the bone of the patient, the bridge spanning the bone fracture, the body temperature of the patient warms the alloy causing it to return to its austensitic form and thus its original shape, with the legs angled inwardly. The staple legs thus exert a compressive force to urge the portions of the bone on opposite sides of the fracture toward each other, which ensures better bone fusion and retards backing out of the staple.
Memory metal can also be super elastic. This unique alloy shows super elastic behavior if deformed at a temperature which is slightly above its transformation temperature. Super elastic bone staples are produced with the staple legs angled inwardly. During insertion of the staple, the staple legs are opened using a staple spreader, and the staple is then inserted into the patient's bone. As soon as the force of the staple spreader is removed from the staple legs, the super elastic metal returns the staple to its original shape with the staple legs angled inward. Again, this results in compressive force on the bone by the staple legs.
Bone stapes today are produced both in shape memory and super elastic forms. In particular, when manufacturing prior art super elastic staples, an ingot of nickel/titanium alloy is first rolled into a flat sheet. The rolling of the metal into a flat sheet results in the formation of grain lines in the metal, much like the grain lines found in wood. The staples cut from the sheets thus have grain lines as well. The shape of the staples cut from the sheet is such that the grain lines of the metal will run generally transverse to the longitudinal axes of the staple's bridge, legs, or both. Grain lines which run transverse or perpendicular to the staple at the intersections between the longitudinal axes of the bridge and/or legs magnify stresses at these intersections in the super elastic staples which can lead to failures or at least reduced ability of the staple to be manipulated freely without fear of failure.